The present invention relates to an electroluminescent device comprising two electrodes, between which there is arranged at least one electroluminescent organic semiconductor layer, and a substrate supporting the said device, as well as an electric current source connected to the electrodes in an electrically conductive manner. The invention also concerns a method of manufacturing such a device.
Within the meaning of the invention, the expression xe2x80x9cat least one electroluminescent organic semiconductor layerxe2x80x9d means an electrically conductive, possibly multilayer, organic material in which an electroluminescence phenomenon may arise when on the one hand electrons and on the other hand positive holes are injected therein. The recombination of these charges with opposite signs causes the emission of light. This is therefore, in the sense of the invention, an electroluminescence said to be by injection.
The phenomenon of electroluminescence using organic semiconductors was revealed for the first time in the 1960s and the development of these electroluminescent systems based on organic thin films dates from the second half of the 1980s. In this regard reference can be made to the following publications: A. L. Kraft, A. C. Grimsdale, A. B. Holmes, Electroluminescent conjugated polymersxe2x80x94Seeing polymers in a new light, Angew. Chem. Int. Ed. (1998) 37, 402-428, and R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Bredas, M. Lxc3x6gdlund, W. R. Salaneck, Electroluminescence in conjugated polymers, Nature/1999/397, 121-128.
In the majority of the cases of the systems used, it is the glass which is taken as a substrate. Successive thin layers constituting the electroluminescent system are deposited on this. More recently, PET (polyethylene terephthalate) has been envisaged for replacing glass. Glass and PET being transparent, indium-tin oxide (ITO) is deposited directly on this substrate, constituting the positive electrode intended, in DC current, to inject positive holes into the organic semiconductor, which is in its turn deposited in one or more layers, possibly consisting of different molecules, on the layer of ITO. Finally, a thin layer of aluminium, magnesium or calcium is deposited on the whole, constituting in DC current the negative electrode intended to inject electrons into the organic semiconductor. It is the hole-electron recombination which generates the light emitted by the system through the glass or PET substrate. In the systems which use alternating current (SCALE: Symmetrically Configured Alternating current Light Emitting devices), the same electrodes are found (ITO on glass or on PET and aluminium, copper or gold) but electrodes no longer necessarily need to have a working function different from each other.
These devices have the drawback that the substrate is a thermally insulating material. During use at high power density this substrate does not allow an appropriate release of heat, which can result in disturbance in the device. In addition, in the case of glass, the substrate is fragile whilst in the case of PET it is flexible. Neither of these two substrates therefore resists the static and dynamic mechanical stresses borne during the use of electroluminescent devices.
Systems are also known which make use of xe2x80x9cphosphorusesxe2x80x9d as a source of electroluminescence. These phosphoruses are inorganic compounds which are separated from a conductive rigid substrate by a dielectric layer, possibly with variable resistance. The phosphoruses are generally encapsulated, for example in a polymerisable resin. They are placed in an alternating electric field which moves the electrons created within them by thermal agitation and the corresponding positive holes created in the valency band. These electrons produce excitations by collision, with the subsequent production of light. This is therefore in this case what is called intrinsic electroluminescence (see for example WO-97/46053 and U.S. Pat. No. 3,626,240).
To excite the xe2x80x9cphosphorusesxe2x80x9d it is necessary to create an alternating field of sufficient intensity, and hence the necessity for the presence of a dielectric and/or resistive layer. The result is high electrical voltages of 60 to 500 V in oscillating alternating current at 50 Hz-2.5 kHz and high thicknesses of approximately 100 xcexcm.
The purpose of the present invention is to develop an electroluminescent device with an organic semiconductor which makes it possible to avoid these problems in a simple fashion.
An electroluminescent device as described at the start has been provided according to the invention, in which the substrate consists of a metal or metallic alloy. Such a substrate has sufficient thermal conductivity to allow discharge of the heat released by the electroluminescent system, especially when the latter is used at high power density.
Advantageously the metallic alloy is a steel, for example soft steel or stainless steel. Steel offers the property of being both rigid and easy to shape, which is advantageous for many applications of electroluminescent devices, such as illuminating panels and external or internal luminaires, decorative systems and fixed or programmable display systems.
According to one advantageous embodiment of the invention, a first electrode is disposed on a first side of the said at least one layer of electroluminescent organic semiconductor, on a first surface thereof which faces the substrate, and a second electrode is disposed on a second side of the said at least one layer of electroluminescent organic semiconductor, on a second surface thereof which is opposite the substrate, this second electrode allowing an at least partial passage of light.
As already mentioned, the device can comprise one or more successive layers of electroluminescent organic semiconductor. First surface and second surface mean, in the case of a single layer of semiconductor, the two faces thereof. In the case of several successive layers, they are the two external faces of this set of layers.
Using a substrate made of metal, metallic alloy or steel advantageously has the effect of allowing a reversal in the arrangement of the layers in the electroluminescent system compared with that of the systems according to the state of the art. This is because the light emitted by the device no longer passes through the substrate but only through one of the electrodes, the one opposite to the substrate, and through any external encapsulation thereof in transparent material, preferably impervious to water and air.
Advantageously, to manufacture this electrode situated opposite the substrate the most transparent possible material is used. It is possible to envisage for example inorganic electrode materials as used in the known electroluminescent or photovoltaic devices for electrodes supported directly by a glass or PET substrate. It is possible to cite, as non-exhaustive examples, indium-tin oxide (ITO), indium-zinc oxide (IZO) or systems based on indium-(zinc, gallium) oxides or ZnO, SnO2, ZnS, CdS, ZnSe, ZnxCd1-xO, ZnTe. It is also possible to use organic transparent electrically conductive materials, such as for example p-doped conjugated polymers, polypyrrole, polythiophene, polyaniline, polyacetylene (CHx) as well as derivatives of mixtures of these substances. It is also possible to make use of several of these superimposed conductive layers, for example a layer of ITO coated with a conjugated polymer.
As a transparent encapsulation material, it is possible to provide by way of example a thin layer of silica deposited for example by the so-called PECVD (Physical Enhanced Chemical Vapour Deposition) technique (SiOx).
According to one advantageous embodiment of the invention, the substrate is connected to the current source. The steel is a good electronic conductor and it can therefore serve as a current feed for one of the electrodes with which it is contact. The substrate can itself serve as an electrode.
It is obviously possible also to provide a device according to the invention in which the substrate supports an electrode which is directly connected to the current source without the current passing through the substrate.
As an electrode material situated on the substrate side, it is possible to envisage any appropriate material for this purpose. Notably the materials indicated above for the electrode situated opposite the substrate can be envisaged. It is however also possible to envisage, as an electrode, the substrate in the form not only of steel sheet itself but more particularly in the form of this sheet which has undergone a surface treatment.
For surface treatment, it is possible to envisage according to the invention any treatment for obtaining superficially in the sheet or on the surface of the sheet a compound which is a good conductor of electricity. It is for example possible to first treat the steel sheet by means of a controlled oxidation so that, at least on the surface, it has a greater proportion of a good conductor, for example Fe3 O4. This controlled oxidation can be designed in a known manner, for example by electrolysis or oxidation in air.
It is also possible to provide, as a surface treatment, the application to the steel sheet of a conductive coating, notably zinc, zinc slightly or greatly alloyed with aluminium, aluminium, chromium or tin. Such coatings can for example be obtained, according to circumstances, by electrolytic deposition or by hot quenching deposition, according to techniques known to experts.
It is also possible to envisage, as surface treatment, the application to the substrate of a thin layer of a metal or alloy other than the one forming the substrate, for example aluminium, magnesium or calcium on a steel sheet. This application can be effected by any means known to experts, for example by vacuum evaporation or cathodic sputtering.
It is possible to envisage the application to the bare substrate, or to the substrate already with surface treatment, of at least one conductive polymer. It is possible to cite, as examples of conductive polymer, polyacetylene, polyaniline, polypyrrole, polythiophene, derivatives thereof and mixtures thereof.
According to one advantageous embodiment of the invention, the substrate is made from steel treated so as to reflect a light emitted from the organic electroluminescent semiconductor layer. The non-transparent steel serving as a substrate can for this purpose be for example polished, as well as its non-transparent coating. It is also possible for the electrode provided on the substrate side and any surface coating of the substrate also to be transparent. Such an arrangement makes it possible to increase not insignificantly the light emission efficiency of the system.
As an electrode material, it is possible to use in particular in this case a material as indicated above with regard to the materials to be used for the electrode situated opposite the substrate.
The replacement of the glass or PET, transparent products, as a substrate by steel, a non-transparent product, makes it possible to use both faces to create electroluminescent devices which are identical or possibly different from one face to the other (changing colour or display).
Other details and particularities of the device according to the invention are indicated in claims 1 to 17. The present invention also concerns a method of manufacturing an electroluminescent device, comprising an arrangement of at least one layer of electroluminescent organic semiconductor between two electrodes, a support for the device by means of a substrate, and a connection of the electrodes to an electric current source. According to the invention, this method comprises an arrangement of a first electrode on a substrate consisting of a metal or metallic alloy, a deposition of said at least one layer of electroluminescent organic semiconductor on the first electrode, and a deposition of a second electrode allowing an at least partial passage of the light on the said at least one layer of organic semiconductor and, possibly a deposition of a transparent material impervious to air and water on the second electrode, so as to encapsulate the device.
Other details and particularities of the method according to the invention are indicated in claims 18 to 24.